Since the early 1900's, efforts have been made to develop an efficient means to convert solid, carbon-containing reactants into liquid fuels. The early work in the field was performed in Germany in the years prior to and between the two world wars. In 1912-13, Frederick Bergius described the fundamental process for hydrogenating coal under very high pressure to yield liquid fuels. (Bergius was awarded a one-half share of the 1931 Nobel Prize in chemistry for this work. Carl Bosch, a titan of the German chemical field, was awarded the other half.) Bergius' “direct liquefaction” of coal was used to produce liquid fuels in Germany during both world wars. A decade after Bergius' work, Franz Fischer and Hans Tropsch, while at the Kaiser Wilhelm Institute, developed the chemistry that now bears their names, and is sometimes referred to as “indirect liquefaction.” The general Fischer-Tropsch synthesis is a metal-catalyzed reaction to produce liquid hydrocarbons from a feedstock comprising hydrogen and carbon monoxide. The feedstock is universally referred to as synthesis gas, or simply “syngas.” The syngas itself is derived from the partial combustion of methane or from the gasification of coal or other biomass. The general reactions are as follows:CH4+½O2→2H2+CO(2n+1)H2+nCO→CnH2n+2+nH2O
The worldwide depression of the 1930's placed a severe economic strain on German companies' early efforts to build large-scale coal gasification plants. As the depression lingered on, crude oil prices plunged to 10 cents per barrel, resulting in a worldwide glut of cheap oil. Two developments, however, stemmed the collapse of the nascent goal gasification industry: (1) the rise of the Nazi government; and (2) the consolidation of the entire German chemical enterprise into an enormous, centrally-organized cartel (I.G. Farben). Begun in 1925, the formation and growth of I. G. Farben and its influence on the development of coal gasification technology can hardly be understated. Underwritten by the Nazi government, and backed by the full might of Germany's preeminent chemical and industrial prowess, German efforts to convert its coal riches into liquid fuel continued unabated throughout the 1930's.
These efforts were vastly expanded during the years of World War II (1939-1945), as Germany was increasingly denied access to sources of crude oil. Synthetic liquid fuels produced from coal gasification accounted for roughly half of Germany's total production of fuel near the end of the war—124,000 barrels per day from 25 plants at its peak near the end of 1944. At that point, synthetic fuel accounted for 92% of Germany's aviation gasoline. (Intense allied bombing of German synthetic fuel plants began in earnest in late 1944 and early 1945. The results were immediate and fatal for the German war machine. In February 1945, Nazi Germany produced roughly a thousand tons of synthetic aviation gasoline—about one half of one percent of the level of the first four months of 1944. Hostilities in Europe ceased in May of 1945.) See U.S. Department of Energy, “The Early Days of Coal Research.”
After World War II, efforts to gasify coal and biomass stagnated as huge reserves of crude oil were discovered and exploited in the Middle East, Venezuela, Nigeria, and elsewhere. The formation of another cartel, the Organization of Petroleum Exporting Countries (OPEC), and its exercise of pricing power in crude oil markets rejuvenated the coal and biomass gasification field. Founded in 1960 by Iran, Iraq, Kuwait, Saudi Arabia and Venezuela (and later joined by Qatar, Indonesia, Libya, UAE, Algeria, Nigeria and Angola), OPEC did not rise to prominence until 1973, when the Arab members of OPEC instituted an oil embargo that sent crude oil prices skyrocketing. The Islamic fundamentalist revolution in Iran in 1979 sent crude oil prices briefly into the stratosphere ($100 per barrel when adjusted for inflation to January 2007). The mid-1980's, however, saw an equally dramatic drop in oil prices from their 1979 highs. Continued political instability in the middle east starting with the 1991 Gulf War, and extending to the panic caused by the Sep. 11, 2001 terrorist attacks in the U.S. (and the subsequent U.S. invasion and occupation of Iraq), coupled with the rapid industrialization of China and India, have combined to maintain current crude oil prices at very high levels.
From a technological standpoint, developments in coal and biomass gasification have proceeded along many fronts. For example, U.S. Pat. No. 2,459,550, issued Jan. 18, 1949, to A. J. Stamm, describes an apparatus for continuous destructive distillation of solids (principally wood in the form of sawdust or chips, and coal in the form of coal dust or pea-sized particles) in a bath of molten metal. The material to be gasified is carried between two finely porous, continuously looped screens that pass beneath the surface of a pool of liquid metal. The heat from the liquid metal is rapidly transferred to the material. Volatile compounds within the material are thereby vaporized, and the vapors pass through the porous screen, rise through the molten metal, and are then condensed. Both the resulting condensate and the charred solid material are then recovered. Similar, single-bath devices are described in U.S. Pat. Nos. 4,649,867; 4,925,532; and 5,693,188.
U.S. Pat. No. 3,647,379, issued Mar. 7, 1972, to Wenzel et al. describes a device for gasifying a coal/water mixture. The device is a single-chamber device in which dehydration of the coal is followed by gasification of the dried coal and then endothermic reaction of the resulting gas products.
U.S. Pat. No. 4,126,668, issued Nov. 21, 1978, to Erickson, describes a method to produce a hydrogen-rich gas such as pure hydrogen, ammonia synthesis gas, or methanol synthesis gas by reacting steam with a non-gaseous intermediate, whereby some of the steam is reduced to hydrogen and some of the intermediate is oxidized. Carbon dioxide may be added to (or substituted for) the steam, whereby carbon monoxide is produced in addition to (or in lieu of) H2. The oxidized intermediate is reduced by a reducing gas. The reducing gas is generated by partially reforming a light hydrocarbon such as natural gas or naphtha with steam and/or CO2, and then partially oxidizing the partially reformed gas with air. The low BTU exhaust gas resulting after reduction of the intermediate oxide is used as fuel for the primary reformer. When ammonia synthesis gas is produced by this process, the purge and flash gases from the ammonia synthesis loop are added to the reducing gas.
U.S. Pat. No. 4,344,773, issued Aug. 17, 1982, to Paschen et al. describes an apparatus for gasifying carbon-containing media. The device includes a molten iron both for gasifying the reactants and a plurality of nozzles for introducing the reactants into the molten iron bath. An outlet is also provided for removing slag from the bath. Because it uses molten iron, this device has distinct drawbacks. Melting the iron requires an extremely high reactor temperature. This, in turn, spawns other considerations. For example, the high temperature of the molten iron is extremely detrimental to the reactor lining. To ensure long lining life requires essentially zero motion of the iron melt.
Likewise, the liquid slag is very difficult to handle due to the extreme temperatures involved. The process also is not energy efficient because it is hard to obtain a quality syngas at such high temperatures.
U.S. Pat. No. 4,345,990, issued Aug. 24, 1982, describes a continuous method for recovering oil and gas from carbon-containing material. The apparatus described here uses two molten-metal baths. No screens are utilized. Instead, the material to be gasified is placed directly into the bath. The first bath is a comparatively low-temperature bath maintained at about 500° C., while the second bath is maintained at a much higher temperature of about 1,200° C. Two different metals, substantially insoluble in each other when melted, are used in the two baths. Lead is the preferred metal for the first bath; iron is the preferred metal for the second bath. The reactant material is deposited into the first bath (molten lead), and the volatized gases are collected. The molten lead, with the partially distilled carbonaceous material within it, is then transferred to the second bath (molten iron). Here, oxygen is injected into the gas space above the molten iron. The carbonaceous material moves from the lead phase, to the iron phase, where it is further volatilized. The volatile gases liberated from the solids react with the oxygen in the headspace above the molten iron. The molten lead (which is not soluble in the molten iron) settles to the bottom of the second bath and is transferred back to the first vessel. Of particular note in this method is that the thermal decomposition in the first bath takes place in the absence of added oxygen, while oxygen is purposefully added in the second thermal decomposition. By recycling the lead that settles to the bottom of the second bath back into the first bath, the heat required to melt the iron is backward integrated to heat the lead too. In the second bath, the remaining amount of carbon in the solid reactant is gasified to syngas by adding a balanced amount of oxygen to the reaction (in the form of oxygen gas, air, oxides, etc.). Any remaining solids are removed as slag. The principal drawback of this device is that it requires pumping molten metals from bath-to-bath. Thus, the device has numerous mechanical parts that operate at extremely high temperatures.
U.S. Pat. No. 5,085,738, issued Feb. 4, 1992, to Harris et al. describes an apparatus for gasifying organic waste materials. The apparatus includes an elongated and inclined chamber filled with molten lead. Organic material introduced in a lower portion of the chamber migrates through the molten lead to a higher portion of the chamber due to the organic material having a specific gravity less than the molten lead. As the organic material migrates through the molten lead, the material is gasified. The resulting vapor-phase hydrocarbons are then captured in a condenser. The gaseous hydrocarbons are utilized to heat the lead in the chamber and the vapor is condensed to liquid hydrocarbons in the condenser. Residual solids flow to a reservoir connected to the chamber. This apparatus described here is intended for processing tire scraps and generally operates in the temperature range of 340° C. to 510° C. Other waste material can be used (such as wood and paper products). However, the pyrolysis products of woody biomass will have high amounts of heavy tar and char at this temperature range. The char would be difficult to manage in this single-chamber reactor apparatus. See also Published U.S. Patent Application 2005/0 131 260.
U.S. Pat. No. 5,478,370, issued Dec. 26, 1995, to Spangler describes a method for producing syngas from lower alkanes. In this approach, a molten metal oxide bath delivers oxygen to a feed stream containing lower alkanes. A reaction thus takes places wherein the lower alkanes are oxidized to produce carbon dioxide and the molten metal oxide is reduced to the elemental metal. The elemental metal is regenerated to the metal oxide by contact with a regenerant such as air. Heat from the molten baths is transferred to an endothermic reactor where a portion of the carbon dioxide-containing gas is converted to a mixture of carbon oxides and hydrogen.
U.S. Pat. No. 6,051,110, issued Apr. 18, 2000, to Dell'Orfano et al. describes a partially integrated, continuous process (and corresponding apparatus) to distill carbonaceous materials. In a fashion similar to the looped screens of the Stamm patent (see above), the Dell'Orfano patent uses mesh baskets to convey the carbonaceous material through the process. Using the baskets also eases recovery of the solid products that remain after gasification. In this approach, the carbon-containing reactants (preferably wood) are passed first through a de-gassing bath containing heated liquefied volatiles recovered from earlier runs (and referred to as “wood petrol” in the patent). The first bath degasses the wood without degrading the released gases. The de-gassed wood is then passed through a molten-metal bath (preferably molten lead), which converts the wood to char and volatiles. The volatiles are collected and a portion of them are recycled for use as the “wood petrol” in the first degassing bath. The remaining gases are collected. Lastly, the char is then passed through a condensing bath. Oxygen is specifically excluded from the second and third baths.
U.S. Pat. No. 6,110,239, issued Aug. 29, 2000, to Malone et al. describes a two-zone process in which a high-pressure hydrogen-rich gas stream and a high-pressure carbon monoxide-rich gas stream are simultaneously produced in separate zones using a molten-metal gasifier. Because the two gas streams are produced in separate zones, this approach eliminates the need to separate or compress the two gases. The process as described includes introducing a hydrocarbon feed into a molten metal bath beneath the molten metal surface in a first feed zone operating at a pressure above five (5) atmospheres absolute, which decomposing the hydrocarbon feed into a hydrogen-rich gas, and carbon. The carbon dissolves in the molten metal. The carbon concentration in the molten metal is carefully maintained to remain at or below the limit of solubility of carbon in the molten metal. A portion of the molten metal is then transferred from the feed zone to another molten metal oxidation zone operating at a pressure above five (5) atmospheres absolute into which an oxygen-containing material is introduced. The carbon dissolved in the metal reacts with the introduced oxygen to form a carbon monoxide-rich gas which leaves the oxidation zone. Thus, the carbon concentration in the molten metal is reduced. In this zone, the carbon concentration in the molten metal is controlled so that it does not reach the concentration at which the equilibrium oxygen concentration would exceed its solubility limit in the molten metal (in which instance a separate iron oxide phase would accumulate). A portion of the molten metal which has a lower carbon concentration from the oxidation zone is then recycled back to the feed zone. The two gas streams are passed out of their respective zones. The main disadvantage of this approach is that the concentration of carbon and oxygen in the two zones must be very carefully controlled, or CO will contaminate the H2 gas stream. If the oxygen exceeds its solubility limit in the second zone of the molten metal, the oxygen will also react with the hydrocarbon in the first zone to create a CO impurity in the hydrogen-rich gases.
U.S. Pat. No. 6,663,681, issued Dec. 16, 2003, to Kindig et al. describes a method for producing hydrogen gas. The hydrogen gas is formed by reducing steam using a metal/metal oxide bath (e.g. iron/iron oxide) to remove oxygen from water. The steam is contacted with a molten metal mixture including a first reactive metal (iron) dissolved in a diluent metal (tin). The reactive metal oxidizes to the corresponding metal oxide, forming a hydrogen gas (via reduction). The metal oxide can then be reduced back to the metal for further production of hydrogen without substantial movement of the metal or metal oxide to a second reactor.
U.S. Pat. No. 6,830,597, issued Dec. 14, 2004, to Green, describes a process and device for gasifying biomass. In this approach, heat from a combustion chamber is used to gasify or liquefy biomass. The combustion chamber partially surrounds a reactor tube and is in direct thermal contact with the reactor tube. In this fashion, heat from the combustion chamber passes directly through the reactor wall to heat the biomass within the reactor tube.
U.S. Pat. No. 6,863,878, issued Mar. 8, 2005, to Klepper et al., describes a method of producing syngas from biomass or other carbonaceous material. The method utilizes a controlled devolatilization reaction in which the temperature of the feed material is maintained at less than 232° C. (450° F.) until most of the available oxygen is consumed. The reaction is carried out at this very low temperature to minimize pyrolysis of the feed material. The method backward integrates the resulting syngas to provide the energy for the initial gasification reaction. The approach does required using high-pressure, high-temperature (1,000° C.) high-pressured steam to gasify the low-temperature biomass residues. This process is inefficient with respect to converting the carbon in the biomass reactant into syngas. The residual air combusts with the feedstock. The resulting energy is used to heat the biomass to the required temperature. That carbon is lost out the flue and is not converted to syngas.
Published U.S. Patent Application 2005/0 032 920, published Feb. 10, 2005, to Norbeck et al., describes a multi-step, integrated, steam pyrolysis apparatus for producing syngas for use as a gaseous fuel or as a feedstock for Fischer-Tropsch reactions. The process is described as “substantially self-sustaining.” Here, slurry of particles of carbonaceous material in water, and hydrogen, is fed into a hydro-gasification reactor under conditions that yield a methane-containing product gas. This methane-containing gas is then fed into a steam pyrolytic reformer to yield syngas. A portion of the hydrogen generated by the steam pyrolytic reformer is fed through a hydrogen purification filter and backward integrated into the hydro-gasification reactor used in the first step. The remaining synthesis gas generated by the steam pyrolytic reformer can be used directly as a fuel. Alternatively, the syngas may be fed into a Fischer-Tropsch reactor to produce liquid fuels. Molten salt loops are used to transfer heat from the hydro-gasification reactor (and the Fischer-Tropsch reactor if a liquid fuel is produced), to the steam generator and the steam pyrolytic reformer.
Very recently, a paper appeared in the Proceedings of the National Academy of Sciences, Agrawal, Singh, Ribeiro & Delgass (Mar. 14, 2007) “Sustainable Fuels for the Transportation Sector,” PNAS, doi:10.1073/pnas.0609921104. This paper presents a much generalized scheme for producing liquid fuels by producing hydrogen (H2) from carbon-free primary energy source, e.g., solar, nuclear, wind. The hydrogen so produces is then reacted with gasified solids, such as coal or biomass. The overall goal is the complete incorporation of every carbon atom present in the reactant into a molecule of liquid fuel product. Carbon dioxide produced in the biomass gasification step is constantly recycled into the reactor, thus eliminating the release of carbon dioxide into the atmosphere. It must be noted, however, that the paper sets forth only a conceptual framework. As the authors themselves state, the chemical processing systems to accomplish the process “are yet to be defined.”
Thus there remains a long-felt and unmet need to produce liquid fuels efficiently from biomass and other solid reactants.